The invention relates to glass bottle testing, and more particularly to a totally mechanical test which eliminates the need for pneumatic or hydrostatic pressure.
Glass bottles designed to hold carbonated beverages are pressure vessels which under certain conditions must withstand substantial pressure. In order to assure the safe performance of each and every glass bottle going into service, some practicable means for testing all of the pressure containers is necessary.
Glass in the form used for the manufacture of bottles is very strong in resistance to compression stresses but very weak in resistance to tension stresses. This is evidenced by the fact that flat glass breaks evenly along a scratch when the side of the glass having the scratch is placed in a small degree of tension. Annealing procedures normally used in the production of glass bottles cause the outside surface of the bottles to assume a state of compression, while the inside surfaces of the bottles are in tension. This effect results primarily from the fact that when a bottle mold opens, the inside surface of the bottle is hotter than the outside surface. The inside surface thus tends to draw the bottle inward upon cooling, shrinking it slightly and placing the outside surface in compression, but of course not shrinking the bottle sufficiently to avoid tension on the inside surface. Bottles so produced are thus able to withstand a certain amount of abrasion and internal pressure without failure, since the outside of the bottle remains in compression.
As is well known in the bottle manufacturing industry, some glass bottles do develop small and even microscopic scratches, checks and abrasions on their outside surfaces between the time they are removed from the mold and the time they are deposited in shipping cases. Often these blemishes are in the bottom because the bottles are transported on their bottoms from one conveyor to another, through annealing ovens and through mechanical, electrical and optical inspection equipment. If such a check, scratch or abrasion goes undetected and the bottle is filled with a product capable of developing pressure, the internal pressure under certain conditions can cause the normal compressive stress on the outside surface of the bottle to be replaced with a tensile stress. Only a small degree of such tensile stress at the location of the blemish will cause the bottle to fail, sometimes causing severe damage and injury. Therefore, to effectively test a glass bottle, critical areas of the bottle's outside surface should be put into tension.
In order to create external tensile stress in a glass bottle during testing, internal test pressure may be used. Compressed air is objectionable, however, since a bottle failing the test will explode with considerable force, throwing glass fragments at high velocity. Compressed air testing is therefore not commonly used for testing because of safety considerations. Even if bottles are enshrouded in connection with compressed air testing to avoid the dangers of flying glass fragments, the procedure is still unsatisfactory since flying glass fragments are extremely abrasive and can quickly wear away any material used to enshroud a bottle.
The general practice in the glass bottle manufacturing industry is presently to withdraw a certain percentage of sample bottles and to test them by filling them with water and subjecting them to hydrostatic pressure. Although this type testing is satisfactory when sample testing is sufficient, it is not satisfactorily adaptable to 100% testing because a considerable amount of time is required to empty the bottle of water after testing, and because some small quantity of water is left inside the bottle even after emptying.
In my co-pending application Ser. No. 497,751, now U.S. Pat. No. 3,895,514, I disclose a bottle tester wherein an expandable bladder is inserted into the interior of the bottle and then filled with water or other liquid to produce a test stress in the bottle walls and bottom. The apparatus provides a reliable test and eliminates most of the problems of prior testing methods, enabling 100% testing to be implemented. However, since air must be removed from the bottle and fluid must be pumped into and out of the bladder, the method and apparatus are not as simple in structure and operation as are those of the present invention described below.